What is JotForm?
JotForm is a free online form builder which helps you create online forms without writing a single line of code. No sign-up required.
At JotForm, we want to make sure that you’re getting the online form builder help that you need. Our friendly customer support team is available 24/7.
We believe that if one user has a question, there could be more users who may have the same question. This is why many of our support forum threads are public and available to be searched and viewed. If you’d like help immediately, feel free to search for a similar question, or submit your question or concern.
I have inspected your form: http://www.jotformeu.com/form/51745043955358 and I see that you have payment field inside of it. When I selected product inside of your form Total and sub-total values are shown.
It is not possible to write in total and sub-total field. Values from other fields are passed to total and sub-total.
Can you please explain in more details what you are trying achieve?
Also this articles can help you to create a payment form.
Hope this will help. Let us know if you need further assistance.